Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

A final note on possession
Apostrophes
Revising at the word level

What you’ve learned in this unit is the simplest system for showing possession. But there are variations to the rules. For example, you usually add an -s to a one-syllable name that already ends in -s, such as James. (In other words, you write about James’s future.) By adding the extra -s, you are supplying a guide to pronunciation. Your teacher may discuss this variation or other ones, but the basic rules you’ve learned in this unit are sufficient to meet the standards of most college teachers.